| Over the legacy time frames experienced by drinking water distribution systems (DWDS), trace inorganic contaminants (TICs) such as arsenic, lead, antimony, thallium, and chromium can accumulate in high concentrations within a biofilm matrix containing iron (Fe) and manganese (Mn) oxyhydroxides. Release of TICs can occur when these matrices are disturbed by a physical or chemical change. A bench scale factorial design using recirculating glass columns (25 mm x 300 mm) with abraded 3 mm glass beads, was used to establish biofilm matrices under a standard conditions at 16°C, 0.2 mg/l chlorine (Cl 2) and 1 mg/L dissolved organic carbon (DOC) for 6 months. The developed matrix was then challenged by altering the temperature (7 and 25°C) and concentrations of Cl2 (0 and 2 mg/L) and DOC (0 and 2 mg/L). During this 4 week challenge phase, effluent TIC concentrations were monitored, after which each column was sacrificed and analyzed for ATP and DNA, and TIC distribution within the solids using sequential extractions and total digestions. Solid phase Mn and Fe were associated with the organic fraction of the biofilm matrix. With increased Cl2, more accumulation of Mn occurred due to the oxidation of Mn(II) to Mn(IV). This resulted in increased surfaces for sorption of Fe and the TICs, Pb, Tl, Cu, and Cr. Increased Cl2 also resulted in a less active biofilm as indicated by lower ATP concentrations within the biofilm matrix. Less As accumulated with increased DOC as a result of surface site competition. Lead was impacted by Cl2 and DOC, where the addition of Cl2 increased the retention of lead while the addition of DOC caused a decrease. The outcome of this investigation was the identification of conditions that cause accumulation of TICs to biofilm matrices under DWDS conditions, demonstrating a high potential for accumulation when the DWDS is sourced with Fe/Mn and TICs at secondary and primary MCLs. Though not the focus of this study, these highly concentrated solids are susceptible to physical disturbance and release showing the importance of both minimizing accumulation and conducting regular pipe cleaning activities. |
